chametz (thing)

Chametz (also transliterated hametz) is the Hebrew term for leavened bread, which is forbidden during the Jewish feast of Pesach (Passover). The prohibition does not apply merely to eating chametz, as it states lo yeira'eh lekha, you may not have chametz "in your borders", i.e. in your possession in any form. This rule is observed strictly
and in the
widest possible sense of the word. To give a few examples:

All observant Jewish families engage in a massive spring cleaning operation before Passover in order to eliminate any bread crumbs that may be lurking in dusty places.

In Jewish shops, all shelves retailing items
that are or contain chametz are covered with sheets to prevent people from accidentally buying any.

Most pet food contains chametz, so observant Jews have to figure out other ways to feed their pets or risk contamination.